This invention described herein was jointly made by employees of the United States Government and by employees of University of Pittsburgh, and it may be manufactured and used by or for the United States Government purposes without payment of royalties thereon or therefor.
1. Field of the Invention
This invention relates to a rotary fluid pump having a magnetically suspended and rotated impeller and a method of making the same. More particularly, this invention concerns a rotary fluid pump comprising a housing, an impeller having magnetic impeller blades, a stator member attached to the housing, means for levitating the impeller such that the impeller is substantially centered within the housing, and means for rotating the impeller, and wherein the geometric configuration of the rotary fluid pump is sized and proportioned to minimize stagnant and turbulent fluid flow.
2. Description of the Related Art
The use of a rotary pump ventricular assist device for aiding a patient""s heart in pumping blood is well known. The rotary pump ventricular assist device may be connected to the patient""s heart in a left-ventricular assist configuration, or a right-ventricular assist configuration, or a bi-ventricular assist configuration. For instance, if the left-ventricular assist configuration is adopted, the rotary pump is connected between the left ventricle of the patient""s heart and the aorta. Generally, the rotary pump comprises a housing having an inlet and an outlet, an impeller positioned within the housing and having impeller blades, and a stator member. The blood enters the inlet of the housing and is pumped by the rotating impeller through the housing to the outlet and into the patient""s circulatory system.
Artificially pumping blood utilizing a rotary pump may be detrimental to the blood. If the rotary pump is inefficient, the pump will impart excessive entropy to the blood which usually takes the form of heat or fracture. The heat produced from the pump can damage the blood. The blood cells may coagulate or the albumin of the blood may denature if the temperature reaches forty-two degrees centigrade (42xc2x0 C.).
Moreover, numerous studies have proven that exposing blood to high stresses results in direct or delayed destruction of blood. As a result of the rotation of the impeller, regions of turbulence, jet formation, cavitation and rapid acceleration may be created and cause the blood cells flowing through the pump to break down and rupture. Also, the geometric configuration of a rotary pump may contribute to regions of retarded flow being formed, such as, recirculation and stagnation which cause blood to deposit on the pump structure resulting in thrombosis.
Many attempts have been made to overcome the above-mentioned disadvantages of utilizing a rotary pump as a ventricular assist device. One type of conventional rotary pump utilizes mechanical bearings that necessitate a lubricant flush or purge with an external lubricant reservoir for lubricating the bearing and minimizing heat generation. Examples of this type of rotary pump are illustrated in U.S. Pat. Nos. 4,944,722 and 4,846,152 issued to Carriker et al. and Wampler et al., respectively. There are many disadvantages to this type of rotary pump. The percutaneous supply of the lubricant purge fluid degrades the patient""s quality of life and provides a potential for infection. Seals for the external lubricant are notoriously susceptible to wear and to fluid attack which may result in leakage and the patient having a subsequent seizure. Also, an additional pump is needed for delivery of the lubricant to the bearing. Yet another disadvantage of this type of rotary pump is that the bearings will need to be replaced over time because of wear due to the bearings directly contacting other pump structure.
In order to eliminate the need for an external purge of lubricant, a rotary fluid pump having a magnetically suspended impeller was created. By utilizing a magnetically suspended impeller, direct contact between the bearing and other pump structures, as well as external lubricant purges are eliminated. Examples of this type of rotary fluid pump are disclosed in U.S. Pat. Nos. 5,326,344 and 4,688,998 issued to Bramm et al. and Olsen et al., respectively. This type of rotary pump generally comprises an impeller positioned within a housing, wherein the impeller is supported and stabilized within the housing by a combination of permanent magnets positioned in the impeller and the housing and an electromagnet positioned within the housing. The impeller is rotated by a ferromagnetic stator ring mounted within the housing and electromagnetic coils wound around two diametrically opposed projections. The ferromagnetic impeller and the electromagnetic coils are symmetrically positioned with respect to the axis of the rotary pump and thus, impose an axially symmetric force on the fluid passing through a single annular gap formed between the housing and the impeller. The disadvantage of this type of rotary pump is that there is only one annular gap for the blood to pass through and it serves competing purposes with respect to fluid flow and the magnetic suspension and rotation of the impeller. Regarding fluid flow, the cap is desired to be large for efficient pumping whereas, for efficient suspension and rotation of the impeller, the gap is desired to be small. In this type of rotary pump, the fluid gap is relatively small and does not allow for efficient pumping of blood therethrough which may result in the destruction of blood cells.
The pursuit of designing a rotary pump which is sized and proportional to satisfy the competing requirements of providing satisfactory hydrodynamic performance and blood bio-compatibility, as well as efficient magnetic levitation and rotation of the impeller, involves the manipulation of numerous design parameters, arguably more than the human designer can manage at one time. The conventional process for designing a rotary fluid pump limits the focus of the design parameters and relies heavily on first order principles, such as Bernoulli""s equation and Euler""s equation, empirical analyses and trial-and-error methods. A prototype of a pump design based substantially on intuition is created and subjected to testing. Only when a fluid exhibiting the characteristics of blood is pumped through the prototype pump is it clear whether the design is viable. Because the cost of building a prototype is usually high and typically multiple prototypes are created and tested before a final, viable pump is completed, the process can be quite expensive and time-consuming. Furthermore, the best design of the infinite number of options is not guaranteed using this process.
Nowhere in the cited related art is there disclosed or suggested a rotary pump for pumping blood through a patient having a magnetically suspended and rotated impeller, wherein the geometric configuration of the pump provides for blood flow that is hydrodynamically and bio-compatibly satisfactory and a method of making the same. Therefore, there is a definite need for a rotary pump having a magnetically suspended and rotated impeller that pumps fluid without creating regions of stagnant and turbulent fluid flow and a method of making the same.
Accordingly, the present preferred invention provides a rotary pump for pumping fluid through a patient having a magnetically suspended and rotated impeller and a pump configuration that minimizes blood trauma and stagnant flow while providing efficient magnetic suspension and rotation of the impeller.
The present preferred invention provides a rotary pump for pumping fluids through a patient substantially comprising a housing, an impeller positioned within the housing and having a plurality of magnetic impeller blades, a stator member, means for levitating the impeller within the housing such that the impeller is substantially centered therein, and means for rotating the impeller, and wherein the geometric configuration of the rotary pump is sized and proportioned to minimize trauma to the blood and stagnant fluid flow through the rotary pump. The plurality of magnetic impeller blades serve the dual purpose of imparting mechanical energy to the blood and providing a flux path for the means of rotating the impeller. The plurality of magnetic impeller blades are preferably a rare earth, high-energy-density type magnet selected from the group consisting of samarium cobalt and neodymium-iron-boron alloy which reduces the effects of magnetic leakage. Alternatively, the impeller blades are made from soft magnetic material such as silicon-iron or cobalt-iron. This material can carry flux densities which are higher than the remanence of the best available permanent magnetic material. Thus, thinner blades can be used for improving blood flow and increasing the efficiency of the motor. Magnets are embedded in the body of the impeller and the impeller blades are attached to these magnets by a flux focusing structure made of soft magnetic material.
The present preferred invention provides for a primary fluid flow region that is large enough to provide for hydrodynamically efficient fluid flow without traumatic or turbulent fluid flow and a magnetic gap which also allows for fluid therethrough without traumatic or turbulent flow and which is small enough to provide for efficient magnetic levitation of the central hub which can be either the stator or the impeller. The magnetic gap can be positioned at the housing or adjacent the hub wherein the hub member can be either the impeller or the stator.
The present preferred invention provides that the individual parts of the rotary pump such as, the impeller and the stator member are designed using a computational fluid dynamics-based design method. Specifically, the geometric configuration of each of the parts of the rotary pump are designed taking into consideration the specific flow characteristics of blood while minimizing trauma, platelet activation and turbulence which are measured by high shear stress with respect to residence time, viscous energy dissipation rate, particle acceleration, negative pressure causing outgassing or cavitation, vorticity, reverse flow (i.e., boundary layer shear locally becoming zero), adverse pressure gradient, the standard deviation of consecutive blade-to-blade axial velocity and boundary layer transport.
The present preferred invention provides an embodiment wherein the stator member has an upstream set of stationary blades and a downstream set of stationary blades, wherein each set of stationary blades serve as magnetic bearing poles. Each of the sets of stationary blades converge around the impeller such that each set defines a magnetic bearing gap across which a magnetic force is applied. This embodiment also provides for the impeller to be substantially axially symmetric having a conical-shaped nose and a conical-shaped tail wherein the converging ends of each of the sets of the stationary blades correspond to the shape of the impeller nose and the impeller tail. Preferably, the impeller blades and both sets of the stationary blades are soft magnetic material and are attached to permanent magnets in the body of the impeller.
The present preferred means for rotating the impeller and means for levitating the impeller employ a mix of electromagnets and permanent magnets in order to minimize the heat generated by the rotary pump that may result in the degradation of blood cells. Preferably, the levitating means comprises a plurality of coils wound around a plurality of backiron segments, magnetic targets positioned on the impeller, a downstream set of magnetic stationary blades and an upstream set of magnetic stationary blades. It is preferred that the levitating means further comprises a plurality of permanent magnets positioned within the backiron segments to create a permanent magnetic bias thus, reducing the steady state current in the plurality of coils. The rotating means can take many forms, such as a variable reluctance motor brushles3 DC motor or an induction motor. Preferably the means for rotating is a brushless DC motor.
The present preferred invention of the rotary pump further provides for a magnetic bearing controller which senses axial and radial movement of the impeller within the housing and repositions the impeller to its centered position within the housing. It is preferred that a controller of minimal complexity be used in which the control is decoupled as follows: (1) linearly transforming the sensors signals electronically or by microprocessor software into five (5) signals corresponding to the x and z motion of the impeller nose, the x and z motion of the impeller tail, and the y motion of the impeller; (2) independently compensating each of these five signals (e.g. proportional-integral-derivative control or magnetic bearing zero-power control); (3) transforming the resulting five signals into current patterns which are summed and applied to the bearing coils wherein the current patterns are chosen such that they result in a force being applied on the impeller which substantially centers the impeller within the housing. For example, the response to a positive displacement in the y direction is a coil current pattern which produces a restoring force in the negative y direction. The linear operation which transforms the sensor signals into the five (5) decoupled displacements and the linear operations which transforms a compensator outputs to coil current patterns preferably is represented as matrix multiplications which are referred to as decoupling matrices. The resulting feedback control may be designed to stably position the impeller in the center of the housing. Related methods are being done by MECOS Traxler, Inc.
The present preferred invention further provides another embodiment of the rotary pump having an impeller with an interior wall defining a void, an exterior wall, outboard blades extending from the exterior wall and inboard blades extending from the interior wall, wherein the stator member extends within the impeller and has stationary blades that are attached to the housing.
The present preferred invention further provides for the rotary pump to be connected to the patient""s heart using an inflow canula having a trumpet mouth nozzle and a substantially hourglass exterior configuration. A second outflow cannula may be attached to the outlet of the housing of the rotary pump. The inflow cannula is intended to minimize leading edge separation between the heart and the rotary pump which can occur with traditional sharp-edged cannula tips. The concave feature of the hourglass configuration assists in the location of the nozzle within the myocardium by placing the nozzle within the heart and then slightly withdrawing the nozzle until a slight resistance is detected.
Other details, objects and advantages of the present preferred embodiments and the method of making the same will become more apparent with the following description of the present preferred invention.